About 70 per cent of NSW voters support changing the law to allow voluntary euthanasia for terminally ill people, the poll of 1647 people in October found.

It comes as the NSW upper house prepares to vote next month on a voluntary assisted dying bill giving the right to people aged 25 or over with "unrelievable suffering" to choose to die with a doctor's help.

About 18 per cent of voters said they were undecided and 13 per cent were opposed or strongly opposed. More than 87 per cent of Nationals voters supported the question, about 61 per cent of Liberal voters and 73 per cent of Labor voters.

The Doctors Reform Society, a think-tank for medical practitioners, will write to all NSW MPs this week arguing legalised euthanasia is particularly important because doctors are growing reluctant to provide once-common forms of end-of-life pain relief. 

"People are dying worse now than they have in the past," said Dr Robert Marr from the society. "The traditional method of using the 'double effect' of morphine to ease pain and indirectly speed death has become much less common."

The society says some doctors report fear of being reported by colleagues opposed to euthanasia if they dispense such treatments, especially as debate on medical ethics is stirred.

But the Australian Medical Association, which opposes medical euthanasia, said it had not heard of any doctors not providing treatment because of reluctance. 

"We are not aware of the issue," a spokeswoman said.

Next month's vote in the Legislative Council is expected to expose divisions in both major parties. With cross-benchers the Shooters, Fishers and Farmers likely to oppose the Bill, MPs predict a tight vote.

If it is passed and referred to the lower house MPs from major parties will also be granted a conscience vote but Premier Gladys Berejiklian and Opposition Leader Luke Foley have said they are personally opposed to the plan.